Two stage heat treatment process for the production of unalloyed, cold-rolled electrical steel

ABSTRACT

A process for the production of unalloyed electro-technical steel strip of improved magnetic and mechanical properties, in which low carbon steel having not more than 0.10 percent carbon is hot-rolled to produce hot-rolled strip covered with hotrolling scale. The scale is then covered with an inorganic hydroxide or carbonate and the strip is heated to 700* and 950*C. for a time sufficient to reduce the carbon content to not more than 0.01 percent. The decarburized strip is pickled, cold rolled to its final gauge, and then subjected to a special two-stage final heat treatment in the first stage of which the temperature is 350* to 600*C., preferably 460* to 490*C., and in the second stage of which the temperature is 600* to 780*C., preferably 680* to 760*C.

United States Patent 1191 Stefan et a1.

[ TWO STAGE HEAT TREATMENT PROCESS FOR THE PRODUCTION OF UNALLOYED, COLD-ROLLED ELECTRICAL STEEL [75] Inventors: Mihaly Stefan; Zoltn Hegediis;

Fiilop Balzs; Gyula Juhasz, all of Budapest, Hungary [73] Assignee: Csepeli Femmu, Budapest, Hungary [22] Filed: Oct. 5, 1972 [21] App]. N0.: 295,250

[30] Foreign Application Priority Data Oct. 21, 1971 Hungary CE 854 [52] US. Cl 148/122, 148/113, 148/121 [51] Int. Cl. 1101f 1/00 [58] Field of Search ..148/122,121,112,113,

[56] References Cited UNITED STATES PATENTS 1,866,925 7/1932 Cioffi 148/122 2,150,777 3/1939 Morrill 148/110 2,327,256 8/1943 Fowle et al. 148/122 2,354,123 7/1944 Horstman et al. 148/113 3,098,776 7/1963 Elarde 148/122 Mar. 11,1975

3,162,553 12/1964 Richards et a1 148/112 3,180,767 4/1965 Easton et a1. 148/121 3,347,718 10/1967 Carpenter et al. 148/111 3,418,710 12/1968 Seidcl ct al. 29/605 3,421,925 1/1969 Hair ct a1 148/112 3,615,903 10/1971 Perry 1411/12..

Primary E.\'aminerWa1ter R. Satterfield Attorney, Agent, or FirmY0ung and Thompson [57] ABSTRACT the first stage of which the temperature is 350 to 600C, preferably 460 to 490C, and in the second stage of which the temperature is 600 to 780C., preferably 680 to 760C.

2 Claims, No Drawings TWO STAGE HEAT TREATMENT PROCESS FOR THE PRODUCTION OF UNALLOYED, COLD-ROLLED ELECTRICAL STEEL This invention relates to a process for the preparation of steel products having improved magnetic and mechanical properties, preferably of bands and sheets,

wherein the hot-formed product is pickled, trans formed to its final dimensions in cold state, subjected to a final heat treatment, and optionally, prior to pickling, the product covered with scales is subjected to heat treatment at a temperature between .700 C and 950 C.

As is known, the stators and cores respectively, of

synchronous motors and direct-current motors of low or medium output are generally made of coldor hotrolled electrotechnical steels containing silicon as alloying element. Sincein asynchronous motors up to an output of kW and in DC motors up to an output of 50 kW the energy loss occuring in the magnetically active part is only about 8 to per cent of the total energy loss, the application of alloyed electrotechnical steels in such motors is not very efficient. The use of unalloyed iron of higher induction is of far greater importance; on the one hand this material is less expensive than steel alloyed with silicon, and on the other hand its greater induction improves the technical parameters of these motors to a considerable extent. It is an important requirement that the unalloyed iron be magnetically isotropic, i.e., its magnetic behaviour be nearly the same in any direction. A further requirement is that the iron should not age magnetically, i.e., the

magnetic properties (coercetive force, magnetic induction etc.) should not decrease upon operating the motors for a longer period.

The preparation of unalloyed electrotechnical iron bands and sheets applied in the stator and core of electric motors still involves considerable technical difficulties.

Namely, the favourable magnetic properties of unalloyed electrotechnical steels can only be achieved if the amount of impurities, first of all that of carbon and oxygen, is very low in the steel used in the production and, furthermore, a special rolling and heat-treating process is required in order to provide high induction and acceptably low iron loss.

The production of the starting steel material of the known unalloyed electrotechnical steel bands requires special care since the oxygen and carbon content of the ingot should be kept simultaneously at the lowest possible level, although, due to the equilibrium conditions, low carbon content would involve higher oxygen content. In order to decrease the amount of inclusions, such steels are generally produced by a converter process and, in order to improve their magnetic properties, generally some phosphorus and/0r manganese is added to the steel as alloying component. The steel ingot of proper composition is pre-heated and hot-rolled to a thickness of about 1.8 to 2.5 mm., thereafter it is picled and rolled to the final thickness (generally about 0.5 to

unalloyed iron band subjected to known heat treatment, e.g. treated by 750 to 780 "C for 2 hours under protecting gas atmosphere or in vacuo, is too soft and its consistency causes several problems when cutting iron cores of complicated shape and design. Namely, in the cutting process the soft band suffers deformation, the cut edge becomes burred and deformed, consequently several difficulties arise upon niounting the cut cores as well as upon winding. At the same time, due

to the deformation, the magnetic properties of the product decline to a considerable extent.

In order to eliminate the above disadvantages. the unalloyed electrotechnical steel bands are marketed by the producers in cold-rolled, hard state as a so-called semifinished product. Cores of appropriate design, constituting the stators and anchors of electric motors are cut in the motor-producing plant from these semifinished bands rolled to their final thickness but not subjected to heat treatment, and the cores are generally subjected to heat treatment after mounting. In order to achieve the desired magnetic properties, heat treatment is carried out at high temperatures, under protecting gas atmosphere for avoiding scale formation.

The main disadvantage of this method is that the production of the the starting material of extreme purity is far more expensive than that of the unalloyed steel bands of usual quality. Most of the difficulties arise from securing low (maximum 0.02 to 0.03 percent) carbon content. With several users further difficulties may arise due to the fact that in order to ensure the desired magnetic properties, products already cut to size should be subjected to the final heat treatment, and electric motor-producing plants generally do not possess a heat-treating furnace working at the appropriate temperature and equipped with protecting-gas source.

lt is also known that the undesirable carbon content of steel can be reduced not only in liquid state during the steel production, but also in solid state. Several processes are known for reducing the carbon content of steel in solid state (so-called decarbonization processes). The simplest of the known processes is the socalled black-annealing which is, in fact, a special heat treatment. According to this known process, prior to cold-rolling, the hot-rolled, scaled .(unpickled) steel band is subjected to heat treatment in the presence of air at a temperature above 700 C, preferably at 800 1.0 mm.) required for further application. This latter step is carried out usually as follows: the sheet is coldrolled to a thickness higher than the final value by about 4 to 12 percent, thereafter the steel is annealed and rolled to the final size by the so-called critical cold treatment.

In order to achieve the required magnetic properties, the steel is to be annealed at the final dimensions. The

C. Under these conditions the carbon content of the steel is oxidized by the oxide content of the superficial oxide layer (scale).

A disadvantage of this process is that decarbonization is time-consuming and under industrial conditions it requires about 24 to 96 hours. Moreover, during the long heat treatment, the surface layer of the metallic iron substance takes up oxide inclusions to different depths due to internal oxidation.

According to another known decarbonization process, by which mainly cold-rolled electrotechnical steel-silicon alloys used for producing transformer-and dynamo bands are prepared, the entire surface of the cold-rolled band at the final dimension and/or at an intermediate dimension of appropriate thickness is exposed to the action of a decarbonizing protecting gas (e.g. of moist hydrogen-nitrogen mixture) which does not oxidize the steelat a temperature of 750 to 960 C. Under these conditions the thin bands are rapidly decarbonized. A disadvantage of this process is, however, that in order to ensure an effective decarbonization, the total surface'of the band should be exposed to the action of the protecting gas; accordingly, for this process expensive furnaces of pullover system and of continuous operation, having relatively high operating costs, are needed. Consequently, the costs of this process increase the price of the unalloyed electrotechnical bands to an unacceptable degree.

This invention aims at a process which enables the reduction of the carbon content of unalloyed semifinished (intermediate) steel products in conventional equipment and thereby renders possible the production of unalloyed electrotechnical steel bands and sheets of good quality and favourable magnetic properties.

This invention is based on the recognition that the time required for the decarbonizing heat treatment (black annealing") of the intermediate steel products covered with scale can be considerably reduced and at the same time, the formation of oxide inclusions due to internal oxidation can also be avoided, if a deposit consisting of certian metal hydroxides and/or carbonates is formed on the surface of the intermediate products prior to heat treatment.

The invention is further based on the discovery that the time required for the above decarbonizing heat treatment can further be reduced and the quality of the obtained intermediate product can further be improved by introducing air or an inert gas of high dew point into the furnace during the heat treatment.

Finally, this invention is based on the discovery that the mechanical and magnetic properties of the product can be improved by carrying out the final heat treatment of the product, cold-rolled to its final dimensions, in a two-stage process, wherein the temperature of the second stage is higher than that of the first one. Under these conditions unalloyed electrotechnical steel bands and sheets of high quality can be prepared even from unalloyed steels suitable only for general purposes.

On the basis of above, the invention relates to a process for the production of steel products, preferably of bands and sheets, of improved magnetic and mechanical properties, wherein the hot-formed product is pickled, transformed to its final dimensions in cold state, subjected to a final heat treatment, and optionally, prior to pickling, the product covered with scale is subjected to heat treatment at a temperature between 700 C and 950 C, characterized in that prior to the heat treatment carried out at 700 to 950 C, a layer of an alkali metal hydroxide and/or alkaline earth metal hydroxide and/or carbonate and/or of aluminium hydroxide is deposited on the surface of the product covered with scales, and/or the final heat treatment is carried out in two steps, wherein the first step of heat treatment is conducted at'350 to 600 C, preferably at 460 to 490 C, and the second step of heat treatment is conducted at 600 to 780 C, preferably at 680 to 760 C.

According to a preferred embodiment of the invention, during the heat treatment of the product covered with scale, air or a protecting gas of a dew point above C, preferably above 50 C, is introduced into the furnace.

The hydroxide or carbonate layer is preferably deposited by immersing the product covered with scale into a solution or suspension containing the appropriate hydroxide and/or carbonate. According to another method of coating, the solution or suspension is blasted onto the surface of the product to be treated. In the case of rolled bands, the solution or suspension can advantageously be deposited by blasting during the winding process.

The most important advantages of the process according to the invention are the following:

a. The rate and the efficiency of decarbonization can be increased by the process to a considerable extent. Thereby the carbon content of a steel band can be decreased to 0.0l percent or less from an initial value of 0.08 to 0.10 percent, without overloading the heattreating capacity. Accordingly, unalloyed, hot-rolled steel bands of conventional composition, prepared in a usual way for general use (as structural steel), can be employed as starting materials for the production of unalloyed electrotechnical steel bands of high quality.

b. Itrenders possible the use of unalloyed, hot-rolled steel bands prepared for general purposes and containing the usual impurities for the preparation of electrotechnical steel bands.

c'. It requires no special equipment and can be carried out, with no additional costs, in heat-treating furnaces, e.g., in bell-furnaces practically available in all of the rolling mills.

d. According to the invention, conventionally produced 'steels of usual purity grade can be used for the preparation of unalloyed electrotechnical steel bands. Moreover, if steels of higher purity grade, prepared by more careful methods, are processed according to the invention, the magnetic properties of the end-product can be improvedto a considerable extent.

e. Due to the excellent mechanical properties of the product prepared according to the invention, the perfect cutting of core sheets of complicated design becomes possible, .andthe magnetic properties of the product hardly decline in the cutting process, so the subsequent heat treatment under protecting gas, which generally causes difficulties at the motor-producing plants, can be omitted.

The process according to the invention is further illustrated by the aid of the following Examples.

EXAMPLE I A sample ofa 2 mm. thick, hot-rolled steel band covered with hot-rolling scale, prepared for the production of general structural steel, containing 0.08 ofC, 0.36 of Mn, 0.02 of Si, 0.027 ofS and 0.010 of P as impurities, is covered with a Ca(Ol-l) layer by immersing the sample in lime milk. After the deposit has dried, the sample is heated at 800 C for 4 hours in a laboratory scale muffle furnace which is electric resistance heated. The heat treatment reduces the carbon content of the sample to 0.007 percent.

A sample of the same band covered also with hotrolling scale is subjected to heat treatment under the same conditions but it is not covered by lime milk deposit. The heat treatment reduces the carbon content only to 0.039 percent.

EXAMPLE 2 A sample of the same band as used in Example I, covered with hot-rolling scale, is immersed in a solution containing 120 g./l. of NaOH and g./l. of Al. When the deposit has dried, the band is subjected to heat treatment under the same conditions as described in Example 1. The heat treatment reduces the carbon content of the sample to 0.009 percent.

EXAMPLE 3 A sample ofa 2 mm. thick band of the same composition as described in Example I and having a hot-rolling scale deposit of at least 20 m/u. thickness, is covered with lime milk as described in Example 1. After the deposit has dried, the sample is heated for 4 hours at 800 C under a vacuum of mmHg. The heat treatment reduces the carbon content of the sample to 0.002 percent.

Example 4 A band roll of 2 mm. thickness, having the same composition as described in Example l, is covered with a Ca(OH) layer without removing the hot-rolling scale, by immersing the loosened band roll several times in a lime-milk solution.

After drying, the coated band roll is treated at 800 C in a bell furnace, ensuring that all the parts of the insert should be heated at 800 C for at least 3 hours. During this treatment air of a dew point of +50 C is blasted under the bell. After the heat treatment the heating bell is removed and the rolls are rapidly cooled. The cooling rate is not critical, but as far as the subsequent special final heat treatment is concerned, the relatively rapid cooling is more advantageous.

Thereafter the rolls are pickled in a to percent sulphuric acid solution in a conventional manner, and are cold-rolled in several passes to a final thickness of 0.70 mm., in a usual way. (If a band of other final thickness is required, the product can be rolled to the appropriate dimension, e.g., to a thickness of 0.5 mm., 0.65 mm., 0.85 mm. or 1.00 mm.).

The band rolled to its final size is subjected to heat treatment in a gas of usual composition (e.g. l6 71 of H 10 7r of CO 10 7: of CO, the rest N at 475 C. All parts of the band should be kept at this temperature for 2 hours. Thereafter the insert is heated to 690 C and kept at this temperature until all parts of the band roll have been exposed to 690 C for 0.5 hours.

After this treatment an easily cuttable band of 0.70 mm. thickness is obtained. The iron loss (V amounts to 4.3 W/kg. The B induction value of the band (an induction measured at a field strength of 25 Oe) is 1.67 tesla.

EXAMPLE 5 The starting material is a hot-rolled band having the same composition as described in Example l. This band is treated essentially in the same way as described in Example/1 with the difference that the heat treatment is carried out in two steps. Accordingly, the band of 2 mm. thickness is first cold-rolled, in several passes, to a thickness of 0.77 mm., the obtained band is annealed in the usual manner at 720 C. and the annealed band is cold-rolled in a single pass to the final thickness of0.70 mm. The obtained band is subjected to the twostage final heat treatment as described in Example 4. The heat-treated band has an iron loss (Vm) of 3.7 W/kg., and an induction (B25) of I68 tesla.

The thus-obtained band is slightly less cuttable than the band obtained by the process of Example 4.

EXAMPLE 6 A hot-rolled band of 2 mm. thickness, containing 0.31 of Si, 0.52 of Mn, 0.06 70 of C, 0.007 72 of S and 0.011 of P as impurities is treated in the way as described in Example 4. The obtained band of a final thickness of 0.70 mm. becomes readily cuttable after heat treatment. The iron loss (V is 3.3 W/kg., and the induction (B amounts to 1.64 tesla.

What we claim is:

l. A process for the production of unalloyed electrotechnical steel strip of improved magnetic and mechanical properties, consisting of hot rolling low carbon steel having not more that 0.10 percent carbon to produce hot-rolled strip covered with hot-rolling scale. then covering said scale with a layer of a member selected from the group consisting of alkali metal hydroxide, alkaline earth metal hydroxide, alkaline earth metal carbonate, aluminum hydroxide and a mixture thereof, then heating the strip with covered scale to 700 to 950C. for a time sufficient to reduce the carbon content to not more than 0.01 percent, then pickling and cold rolling to its final gauge the decarburized strip, and thereafter heating the strip in a first stage to 350 to 600C. for about 2 hours and in a second stage to 600 to 780C. for about 0.5 hour.

2. A process as claimed in claim 1, in which saidfirst stage temperature is 460 to 490C. and said second stage temperature is 680 to 760C. 

1. A PROCESS FOR THE PRODUCTION OF UNALLOYED ELECTROTECHNICAL STEEL STRIP OF IMPROVED MAGNETIC AND MECHANICAL PROPERTIES, CONSISTING OF HOT ROLLING LOW CARBON STEEL HAVING NOT MORE THAT 0.10 PERCENT CARBON TO PRODUCE HOT-ROLLED STRIP COVERED WITH HOT-ROLLING SCALE, THEN COVERING SAID SCALE WITH A LAYER OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HYDROXIDE, ALKALINE EARTH METAL HYDROXIDE, ALKALINE EARTH METAL CARBONATE, ALUMINUM HYDROXIDE AND A MIXTURE THEREOF, THEN HEATING THE STRIP WITH COVERED SCALE TO 700* TO 950*C. FOR A TIME SUFFICIENT TO REDUCE THE CARBON CONTENT TO NOT MORE THAN 0.01 PERCENT, THEN PICKLING AND COLD ROLLING TO ITS FINAL GUAGE THE DECARBURIZED STRIP, AND THEREAFTER HEATING THE STRIP IN A FIRST STAGE TO 350* TO 600*C. FOR ABOUT 2 HOURS AND IN A SECOND STAGE TO 600* TO 780*C. FOR ABOUT 0.5 HOUR.
 1. A process for the production of unalloyed electro-technical steel strip of improved magnetic and mechanical properties, consisting of hot rolling low carbon steel having not more than 0.10 percent carbon to produce hot-rolled strip covered with hot-rolling scale, then covering said scale with a layer of a member selected from thE group consisting of alkali metal hydroxide, alkaline earth metal hydroxide, alkaline earth metal carbonate, aluminum hydroxide and a mixture thereof, then heating the strip with covered scale to 700* to 950*C. for a time sufficient to reduce the carbon content to not more than 0.01 percent, then pickling and cold rolling to its final gauge the decarburized strip, and thereafter heating the strip in a first stage to 350* to 600*C. for about 2 hours and in a second stage to 600* to 780*C. for about 0.5 hour. 